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	<PARAM NAME="Keyword" VALUE="Train acceleration">
	<PARAM NAME="Keyword" VALUE="Acceleration">
	<PARAM NAME="Keyword" VALUE="Train braking">
	<PARAM NAME="Keyword" VALUE="Braking">
	<PARAM NAME="Keyword" VALUE="Light braking">
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<P><B><A NAME="6.5"></A>6.5  Train performance</B></P>

<P>Trains accelerate and brake according to the characteristics that are set in the timetable and conditions on the track.  The power that is quoted represents gross power, but only 80% of that power is available at the wheels because of internal losses.  Acceleration always assumes the application of maximum power. </P>

<P>Braking force is the maximum available force available to reduce the speed of the train.  It's expressed in the same units as the train mass - metric tonnes - and it establishes the maximum rate of deceleration that is possible.  The maximum deceleration rate can be found from the ratio of braking force in tonnes to train mass in tonnes, where a value of 1 (i.e. 100%) equals -1g, where g is the rate of acceleration due to gravity.  The maximum brake force allowed by RailOS is the train mass, corresponding to a deceleration rate of -1g where a train travelling at 200km/h would come to rest in 5.7 seconds and travel only 157 metres.  It's unlikely that any train would have such a high value, and a value around 10% or 15% is more realistic.  A value of 10% gives a stopping time from 200km/h of 57 seconds and distance travelled of 1.57km. The rate would normally be higher for rapid acceleration/deceleration suburban or underground trains and lower for heavy freights.</P>

<P>Braking force expressed in tonnes sometimes causes confusion because tonne is a unit of mass rather than force, but it's a convenient value to use and one tonne force represents the force that earth's gravity exerts on a mass of one tonne.  The formula that links the two is given by 
Braking force = (Mass x Deceleration)/9.81, where Braking force and mass are both expressed in tonnes, deceleration is expressed in metres per second per second, and 9.81 is the gravitational constant also expressed in metres per second per second.  This is valid for all terrestrial railways but not for railways elsewhere in the universe!</P>

<P>Normal braking uses up to half the maximum braking effort provided that there is time to respond, and calculations are made when the train reaches the end of each track element.  This explains why a train that has reached the element immediately preceding a red signal comes to a complete stop before accelerating again, even if the signal changes from red before the train has stopped.  The braking calculation is done by first calculating the maximum speed that the train can have at the end of the next element in front, then looking forward for the distance from that point that is needed to come to a stop using half the maximum braking effort.  If no reason to brake is found in that distance then the train accelerates if it can, or stays at the same speed if it can't.  If there is a reason to brake then the braking rate is calculated to achieve the necessary stop or speed restriction in the distance needed.  Under normal circumstances there should never be a need to brake at more than half the maximum rate, but if signals are reset in front of a train or points are changed that direct the train into a siding then much heavier braking will be needed.  The floating window indicates the braking rate that is applied - light braking up to 50%  of maximum, heavy braking from 50% to 90% of maximum, and emergency braking above 90%.  When emergency braking is used there is a very good chance that the train will fail to stop in the required distance.</P>

<P>When a train enters the railway from outside or is created on the railway, then the starting speed will be as set in the timetable unless there is a reason to reduce it, for example because of adverse signals.  This means that a new train should never pass a signal at danger or crash into buffers because of excess starting speed, but it may crash into an existing train if the new train isn't protected and the existing train is in its way.</P>

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